The thermoplastics are known to exhibit high resistivity to corrosive chemicals. As a result, thermoplastic pipes are widely employed in chemical and petrochemical industries and in industries that use or produce highly corrosive materials as part of manufacturing processes. The particular thermoplastic material used in a pipe system will be selected in accordance with the anticipated range of chemicals to be carried through the pipes. For example, polyvinyl chloride (PVC) resists attack by most acids and strong alkalis as well as gasoline, kerosene, aliphatic alcohol, hydrocarbons and salt solutions. However, aromatic, chlorinated organic compounds and lacquer absolvents do have an effect on PVC. On the other hand, polyvinylidene fluoride (PVDF) has superior chemical resistance and is used in many situations where PVC would not be appropriate. PVDF also has a high molecular weight fluorocarbon that exhibits superior abrasion resistance and desireable dielectric properties and mechanical strength. PVDF maintains its desireable characteristics over a very broad temperature range, and is used in systems carrying chlorine, bromine and other halogens, most strong acids and bases, aliphatics, aromatics, alcohols and chlorinated solvents. PVDF, however, is not recommended for ketones or esters. Chlorinated polyvinyl chloride (CPVC) is a thermoplastic with characteristics similar to PVC, but with a working temperature range that is higher than PVC. Another thermoplastic used in pipe systems is polypropylene (PP) which possesses excellent chemical resistance to many acids, alkalies and organic solvents. However, PP is not recommended for use with chlorinated hydrocarbons and aromatics. Still other thermoplastic materials used in piping systems include acrylonitrile butadiene styrene (ABS) and glass-filled polypropylene (PPG).
The components of thermoplastic pipe systems are fused to one another by an appropriate application of heat and pressure. The heat causes the thermoplastic to soften temporarily, and the softened edge or surface regions are then urged into direct abutting contact. The abutting surfaces or edges then will harden and integrally join with one another to provide a leak proof connection. However, one type of thermoplastic material generally will not fuse to another type of thermoplastic material. Consequently engineers typically have been required to select a particular thermoplastic material based upon the nature of the chemicals to be carried, and then will design an entire system or subsystem with the selected thermoplastic.
It should be noted that the costs of different types of thermoplastic materials vary significantly. Thus, a pipe system formed from CPVC or PVDF may cost more than a corresponding system formed from PVC.
Thermoplastic pipe systems are widely employed to carry very hazardous chemicals. A leak from a pipe system carrying such chemicals could be environmentally catastrophic, and could cause at least local health problems. To avoid this potential, double containment thermoplastic pipe systems have been developed and are widely employed. The double containment pipe system includes at least one inner carrier pipe disposed within an outer containment pipe. The outer containment pipe functions as a fail safe which at lest temporarily contains any hazardous material that may leak from an inner carrier pipe. A double containment pipe system may be employed with sensing means which is operative to identify the existence of a fluid within the space between the inner carrier pipes and the outer containment pipe. For example, the sensor may be a sensor wire that generates a signal in response to contact by a fluid. The sensor wire may extend longitudinally in the space between the inner carrier pipe and the outer containment pipe. Other systems employing different types of sensors also are known.
An extremely effective and efficient double containment pipe system is shown in U.S. Pat. No. 4,786,088 which issued to Christopher G. Ziu on Nov. 22, 1988 and which is assigned to the assignee of the subject invention. A divisional of the above-identified U.S. Pat. No. 4,786,088 resulted in U.S. Pat. No. 4,930,544 which issued on Jun. 5, 1990 and also is assigned to the assignee of the subject invention. The double containment thermoplastic pipe assembly shown in these two patents includes an inner carrier pipe supported generally concentrically within an outer containment pipe by a plurality of supports. U.S. Pat. No. 4,930,544 is specifically directed to a restraint coupling for use in such a system. In particular, the inner carrier pipe may carry very hot fluids that will cause the pipe to periodically expand. The expansion of the inner carrier pipe over a great length could cause the inner carrier pipe to buckle into the outer containment pipe or to expand longitudinally into the outer containment pipe at an elbow or other such fitting. The restraint coupling disclosed in U.S. Pat. No. 4,930,544 prevents that problem by rigidly fixing the inner carrier pipe to the outer containment pipe at selected locations along their lengths. In particular, the restraint coupling of U.S. Pat. No. 4,930,544 is unitarily formed from a thermoplastic material and includes a generally cylindrical containment portion defining a diameter substantially equal to the diameter of the containment pipe and a generally cylindrical carrier portion defining a diameter substantially equal to the diameter of the carrier pipe. A connecting portion extends rigidly between the containment and carrier portions of the restraint coupling. The containment and carrier portions of the restraint coupling shown in U.S. Pat. No. 4,930,544 define longer axial lengths than the connecting portion. Thus, the carrier portion of the restraint coupling can be butt fused in end-to-end relationship to a carrier pipe of the pipe system, and the containment portion of the restraint coupling can be butt fused in end-to-end relationship with the containment pipe of the pipe system.
Although the restraint coupling and the overall double containment piping system shown in U.S. Pat. No. 4,930,544 has performed exceptionally well and has received very substantial commercial acceptance, the restraint coupling is substantially limited to double containment pipe systems where the inner carrier pipe and the outer containment pipe are formed from the same type of thermoplastic. In particular, the restraint coupling shown in U.S. Pat. No. 4,930,544 is unitarily formed and hence includes the same thermoplastic material on the carrier and containment portions thereof. This thermoplastic material must be compatible with the thermoplastic of both the inner carrier pipe and the outer containment pipe of the pipe system, thereby requiring the inner and outer pipes of the double containment pipe system to be formed from the same thermoplastic as well. As noted above, however, there are very substantial cost differences for different types of thermoplastic materials. For example, a fairly costly inner carrier pipe formed from a PVDF thermoplastic may be required in view of the high temperature fluids to be carried through the pipe. The prior art system necessarily would require the outer containment pipe to be formed from the same fairly costly PVDF. However, any leakage that may occur would cool rapidly, and hence the more costly PVDF thermoplastic might not necessarily be required for efficient containment. Nevertheless, the more costly PVDF would have to be employed to achieve compatibility with the restraint coupling.
In view of the above, it is an object of the subject invention to provide a restraint coupling that enables an inner carrier pipe to be formed from a different material than the outer containment pipe.
It is another object of the subject invention to provide a restraint coupling that enables an economically more efficient double containment pipe system.
Still a further object of the subject invention is to provide a restraint coupling formed from a thermoplastic material for welding to a less expensive outer containment pipe but for restraining an inner carrier pipe formed from a more expensive material.